Pathology Advisory Note
(No. 11)
De-icing salt
damage to trees
De-icing Salt
Damage to Trees
Joan F Webber, David R Rose,
Martin C Dobson
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De-icing Salt Damage
Introduction
Rock salt, sodium chloride (NaCl), has been used in increasing quantities since the Second
World War for de-icing roads and paths. Unfortunately, these continuing annual
applications of salt can have major detrimental effects on the environment, affecting
water-courses, plants and especially trees. Following hard winters, large quantities of salt
are used and the resulting damage to trees can be considerable. In 1971 it was
estimated that de-icing salt was directly responsible for the deaths of over 700 000 trees
annually in Western Europe (Flückiger & Braun, 1981), and more salt is used to de-ice
thoroughfares in Britain than in any other European country. In the past, during severe
winters in Britain (such as in 1979–80 and 1981-82), the total application of salt per
square metre of roadway could easily exceed 5 kg. Since then, improvements in the
calibration of equipment and better predictions of icy conditions have dramatically
reduced the amount of salt used. But the risk of high rates still exists from repeated
applications and from hand spreading.
Figure 1. UK snowfall since 1961 (provided by UKRLG, 2009)
Figure 1 shows that since the winter of 1990/91 annual snow fall reduced consistently.
This played a part in reducing salt use as well as the improvements in the techniques of
salt spreading. It also led to a reduction in the size of salt stocks held at the various
depots throughout the country. However, recent severe winters have seen a dramatic rise
in the use of de-icing salt such that stocks began to run low. Use on roads had to be
reduced by 25% in order to conserve stocks and ensure that major roads were kept clear.
The reduction was necessary in both 2008/09 and 2009/10 as the snow and frost periods
were much longer than in recent years. The winter of 2008/9 was the snowiest since 1991
and 2009/10 was the snowiest since 1978/9; 2010/11 acknowledged as the coldest for 31
years with winter snows starting very early in November.
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Symptoms on deciduous trees
Salt damage can occur either through salt-contaminated soil or through salt spray. The
symptoms which result can affect entire trees, or may just be confined to individual
branches.
Soil salt
Damage from salt-contaminated soil occurs most frequently in urban areas where large
amounts of salt are used for de-icing roads and pavements. Trees close to roads can be
directly exposed to splash, runoff and ploughed snow which can contain considerable
quantities of salt. This salt is taken up by the roots and subsequently transported to the
shoots.
In deciduous tree species the chloride from the salt tends to accumulate in dormant twigs
and buds, with concentrations peaking just prior to budburst (Hall et al., 1972; Lumis et
al., 1976). Depending on the sensitivity of a particular species and the concentration of
accumulated Cl-, often the first visible result in spring is the failure of the buds to open
and so entire branches are devoid of leaves (Hofstra et al., 1979). Alternatively the buds
may open but the developing leaves then wither and die, a process known as ‘post
flushing dieback’ (Gibbs & Burdekin, 1983). Typically, these dead leaves then remain
attached to the shoot (Figure 1).
Figure 1. Dead, partially flushed leaves of beech still present in July
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When buds die before or shortly after flushing, leafing-out may occur later in the season
from dormant or adventitious buds and may give branches a ‘tufted’ appearance.
Alternatively, if the salt damage is less severe, the buds may flush up to 3–4 weeks late
but the leaves can remain small for the rest of the growing season.
Symptoms of salt damage may also develop on apparently healthy branches or trees
where initially foliage developed normally. However, leaves may begin to show marginal
browning and necrosis from June onwards and then wrinkle and curl. The necrosis then
extends into the inter-veinal tissue so by August trees can have an autumnal appearance
with premature leaf fall by the end of September (Figure 2a).
With very severe salt damage, crown dieback including leaf, shoot and limb death is not
unusual. This may occur very quickly over a few months and lead to the death of whole
trees (the latter is especially common on young and newly planted trees), or it may
involve a more gradual decline.
a
b
Figure 2. Symptoms of salt damage: (a) Fully developed leaves of London plane with
marginal and inter-veinal necrosis as a result of salt contaminated soil; (b) Tip-burn
of needles of Monterey pine caused by salt spray
Salt spray
Trees can also sustain damage from salt spray, most noticeably along motorways and
trunk roads with fast moving traffic. Dormant twigs intercept the salt which may then
reach living tissue by entering via leaf scars (Sucoff, 1975). This results in the death of
buds and may cause dieback of the previous years twig growth. Due to the death of apical
buds, lateral buds on wood more than one year old may be released leading a ‘witches
broom’ appearance. However, in contrast to damage from salt in the soil, marginal
necrosis of leaves rarely occurs. It is also rare for salt spray to cause the outright death of
trees, but annually recurring damage tends to keep crowns narrow and stems thin.
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Symptoms on evergreen trees
Symptoms associated with soil-salt toxicity are generally similar to those from salt spray.
One indication of soil salt damage rather than salt spray damage is the failure of buds to
flush and the browning of needles in the summer that they emerge (Sury & Flückiger,
1983). Another indicator of damage from salt in the soil is when needles in the upper
crown show proportionately greater damage than those lower down.
Damage from salt spray first becomes apparent on one-year-old needles as ‘tipburn’
(Spotts et al., 1972). The tips of the needles first turn yellow, then bronze and
subsequently become brown and necrotic (Figure 2a). There is generally a clear
demarcation between the basal green and the terminal brown tissue. Through spring and
summer the necrosis progresses towards the base of the needle. Buds of pines are rarely
affected by salt spray and flush normally in the spring with new needles remaining green
and healthy throughout their first year.
Practical diagnosis of salt damage
Some of the symptoms described above can be caused by stresses other than salt. For
example, marginal necrosis of leaves can also be due to drought, or desiccation by strong
winds. Therefore a thorough evaluation of the symptoms and consideration of all the
environmental conditions at a particular site is needed before de-icing salt can be singled
out as the cause of damage. Location of the injured tree and distribution of damage within
a tree are valuable guides when assessing the possible contribution of salt to damage of
roadside trees. Tree species also vary considerably in susceptibility to salt damage (Figure
3) and the appendix lists the relative susceptibility of most common tree species.
Figure 3. Tolerance to salt - shoots of Pinus nigra (tolerant) and Pinus contorta
(intermediate)
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The following are general injury patterns that have been identified and may apply equally
to salted paths and pavements as to roads (see also Dobson (1991) for more details).
• Trees close to roads are generally the worst affected. Damage is most severe within
5 m of the road but there is frequently a distinct injury gradient with distance;
damage becoming minimal at about 30 m from the road. Keep in mind however, that
where roots penetrate drains carrying salty runoff damage may occur at a
considerable distance from roads.
• Trees on the downhill side of a road suffer more damage than those on the uphill
side.
• Trees planted in depressions or with depressions around their base (e.g. where
planting soil has settled) suffer more damage than trees in raised planting sites.
On high-speed roads (e.g. motorways), where salt spray rather than runoff is the major
cause of injury, the following distribution of damage can be seen.
• Trees on the downwind side of the carriageway show greatest injury.
• Injury is greatest on the side of the tree facing the road.
• Trees sheltered from spray, e.g. by fencing between trees and road, lack injury
symptoms.
• Injury is greatest on the lowest branches. Branches above the spray-drift zone are
not injured or are less injured.
• Flowers may only come out on the side of the tree facing away from the road. This is
because flower buds are more sensitive to salt spray than leaf buds.
• Where deep snow lies for significant periods of time (which rarely occurs in most
parts of Britain), injury does not occur beneath the snow line.
Where foliage is showing signs of salt damage, analysis of chloride levels within the
foliage can confirm that salt was involved. Gloves should be worn when collecting
samples for analysis to avoid any contamination of the sample by salt present in
sweat on the skin surface.
Conclusions
Considerable progress has been made in the recognition of salt damage symptoms since
the first investigations in the 1950s so that general patterns of injury are now fairly well
understood. There is also a substantial body of data in the literature concerning the
concentrations of sodium and chloride associated with foliar injury.
Over the years salt usage has reduced as it is now stored and spread more efficiently. The
lack of de-icing salt damage to trees after the 2008/9 winter was notable as this was the
snowiest winter for 18 years. Likely reasons for this are that the quantity of salt used was
much lower than in previous bad winters and also the spreading techniques have been
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vastly improved. However with prolonged periods of snow and ice, as have been
experienced in the recent winter of 2009/10, there is still a possibility that salt can
accumulate in the soil to levels that might cause severe injury to trees. Apart from this,
the main risk of salt injury exists in areas where salt has to be spread by hand. In these,
and other sensitive areas, the use of alternative de-icing materials such as urea and CMA
(calcium magnesium acetate) has been used to great effect to prevent damage to trees.
D. Rose and J. Webber
Centre for Forestry and Climate Change, Forest Research
This Note was adapted from Forestry Commission Bulletin 101: De-icing Salt
Damage to Trees and Shrubs by M C Dobson
References
Dobson, M.C. (1991) De-icing salt damage to trees and shrubs. Forestry Commission
Bulletin 101. HMSO.
Flückiger W. & Braun S. (1981) Perspectives of reducing the deleterious effects of
de-icing salt upon vegetation. Plant and Soil 63, 527-529.
Gibbs J.N. & Burdekin D.A. (1983) De-icing salt and crown damage to London plane.
Arboriculture Journal 6, 227-237.
Hall R., Hofstra G. and Lumis G.P. (1972) Effects of de-icing salt on eastern white
pine: foliar injury, growth suppression and seasonal changes in foliar concentrations
of sodium and chloride. Canadian Journal of Forest Research 2, 244-249.
Hofstra G., Hall, R. and Lumis, G.P. (1979) Studies of salt induced damage to
roadside plants in Ontario. Journal of Arboriculture 5, 25-31.
Lumis G.P., Hofstra G. & Hall R. (1976) Roadside woody plant susceptibility to
sodium chloride accumulation during winter salting. Canadian Journal of Plant
Science 56, 853-859.
Spotts R.A., Altman J. and Staley, J.M. (1972) Soil salinity to ponderosa pine
tipburn. Phytopathology 62, 705-708.
Sucoff E. (1975) Effects of deicing salts woody plants along Minnesota roads.
Minnesota Agricultural Technical Bulletin 303.
Sury R. & Flückiger W. (1983) The effect of different mixes of NaCl and CACl2 on the
silver fir (Abies alba Miller). European Journal of Plant Pathology 13, 24-30.
UKRLG (2009). Lessons from the Severe Weather February 2009. UK Roads Liaison
Group, February 2009
http://www.ukroadsliaisongroup.org/pdfs/UKRLG%20Report%20Final.pdf
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Appendix:
Tolerance to soil salt of common tree species
Tolerance
Species
Tolerant
Alnus glutinosa
Tolerant
Elaeagnus angustifolia
Tolerant
Gleditsia triacanthos
Tolerant
Pinus nigra (all varieties/subspecies)
Tolerant
Picea pungens
Tolerant
Quercus robur
Tolerant
Robinia pseudoacacia
Tolerant
Salix alba
Tolerant
Ulmus glabra
Intermediate
Acer campestris
Intermediate
Alnus incana
Intermediate
Crataegus monogyna
Intermediate
Carpinus betulus
Intermediate
Fagus sylvatica
Intermediate
Fraxinus excelsior
Intermediate
Picea abies
Intermediate
Pinus contorta
Intermediate
Pseudotsuga menziesii
Intermediate
Sorbus aucuparia
Intermediate
Thuja occidentalis
Sensitive
Acer pseudoplatanus
Sensitive
Aesculus species
Sensitive
Betula pubescens
Sensitive
Cornus species
Sensitive
Corylus species
Sensitive
Larix decidua
Sensitive
Platanus x hispanica
Sensitive
Prunus avium
Sensitive
Tilia cordata
Sensitive
Tilia platyphyllos
Published by Forest Research
© Crown Copyright 2011
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